Plasmons are an aggregate oscillation of charge density in a material. In the case of the charged graphene sheet, we get surface plasmons from the oscillations of the charged added to the sheet as mentioned in the article.

Plasmons are kind of cool because they can associate with photons to create polaritons. It's not clear to me without having read the paper if they're proposing that polaritons be induced or if the emissions are being caused by spontaneous plasmons in the graphene.